Handle a missing encrypted_key or iv for keygen

Previously if you tried to use encrypted_key/encrypted_iv and had not
loaded them into the ENV (or wherever they will come from) yet, you
would not be able to generate new keys in that environment using ```rails generate
symmetric_encryption:new_keys``` because the Cipher would error out.

This change handles missing encrypted_key or encrypted_iv the same way
we handle missing files when using key_filename or iv_filename in a
given environment.

This way you can actually run the generator in the target environment
without it blowing up.